Re: [Qemu-devel] [PATCHv4 5/9] migration: search for zero instead of dup pages

[Orit Wasserman <owasserm@redhat.com>]

On 03/22/2013 02:46 PM, Peter Lieven wrote:
> virtually all dup pages are zero pages. remove
> the special is_dup_page() function and use the
> optimized buffer_find_nonzero_offset() function
> instead.
> 
> here buffer_find_nonzero_offset() is used directly
> to avoid the unnecssary additional checks in
> buffer_is_zero().
> 
> raw performace gain checking zeroed memory
> over is_dup_page() is approx. 15-20% with SSE2.
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  arch_init.c |   21 ++++++---------------
>  1 file changed, 6 insertions(+), 15 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index 1b71912..9ebca83 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -144,19 +144,10 @@ int qemu_read_default_config_files(bool userconfig)
>      return 0;
>  }
>  
> -static int is_dup_page(uint8_t *page)
> +static inline bool is_zero_page(uint8_t *p)
>  {
> -    VECTYPE *p = (VECTYPE *)page;
> -    VECTYPE val = SPLAT(page);
> -    int i;
> -
> -    for (i = 0; i < TARGET_PAGE_SIZE / sizeof(VECTYPE); i++) {
> -        if (!ALL_EQ(val, p[i])) {
> -            return 0;
> -        }
> -    }
> -
> -    return 1;
> +    return buffer_find_nonzero_offset(p, TARGET_PAGE_SIZE) ==
> +        TARGET_PAGE_SIZE;
>  }
>  
>  /* struct contains XBZRLE cache and a static page
> @@ -443,12 +434,12 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
>  
>              /* In doubt sent page as normal */
>              bytes_sent = -1;
> -            if (is_dup_page(p)) {
> +            if (is_zero_page(p)) {
>                  acct_info.dup_pages++;
>                  bytes_sent = save_block_hdr(f, block, offset, cont,
>                                              RAM_SAVE_FLAG_COMPRESS);
> -                qemu_put_byte(f, *p);
> -                bytes_sent += 1;
> +                qemu_put_byte(f, 0);
> +                bytes_sent++;
>              } else if (migrate_use_xbzrle()) {
>                  current_addr = block->offset + offset;
>                  bytes_sent = save_xbzrle_page(f, p, current_addr, block,
> 
Reviewed-by: Orit Wasserman <owasserm@redhat.com>